Viruses

Viruses, viroids, and prions are all acellular pathogens. They are not within any kingdom and carry their own significant characteristics.

obligate intracellular parasite – require a host to cause damage

filterable – small enough to be filtrated

contains an outer protein coat and inner genome

has only 1 kind of nucleic acid (RNA or DNA, but never both)

lacks metabolic abilities

Viruses can be found either inside a cell (intracellular) or outside of a cell (extracellular). If it is found extracellular, the virus is called a virion. A virion contains a protein coating called a capsid, which surrounds the core of the virus containing the nucleic acid (either DNA or RNA). Together with the capsid and the DNA or RNA core is called a nucleocapsid. Some virions also contain an envelope which is made up of a phospholipid membrane. Both the capsid and the envelope are important in protection and providing shape to the virus.

How are they Classified?

structure

host that they infect

plant

animal

RNA/DNA viruses

bacterial viruses (also known as bacteriophage) because the bacterial host cell is eaten up by the virus cell

their size

nature of the capsid coat

whether it has an envelope or not

Genome of the Virus

Are either single or double stranded DNA or RNA. The overall size of it is much much smaller than that of bacteria.

Host of Viruses

Specialists. Some viruses only affect a certain kind of host, animal, plant etc, and kind of cells within that host. So viruses infecting HIV patients only the helper T lymphocytes (helper T cells, responsible for the communication among other immunity cells) specifically because of the glycoprotein 120 (gp120). Glycoproteins are seen at the surface of the T helper cells. Human immunodeficiency virus (HIV) are able to recognize the GP120 and binds to the GP120.

Generalists. Some viruses infect a variety of cells in different hosts, like rabies.

Size of a Virus

They range from the smallest being 10nm to the largest of 400 nm

Capsid

Protein coat containing capsomeres.

Viral Shapes

Helical

Polyhedral – most common is the icosahedron (20 Sides)

Complex – These are neither helical or polyhedral, example is the bacteriophage

Viral Envelope

Although viruses lack cell membrane, some viruses have a viral membrane surrounding its capsid. These are called enveloped virion. One without an envelop is called a naked or nonenveloped virion.

Virus gains the envelope from their host. Envelopes are made up of proteins, phospholipid bilayer, and some glycoproteins protruding out (looking like spikes) at the surface of the virus.

Viroids

These are very small, circular RNA (may appear linear), and infectious in plants. They do not contain a capsid.

Prions

Prions are proteinaceous infective particles. Prions do not contain nucleic acid. Prions contain a single protein called PrP. The sequence of amino acid that makes up PrP can fold into at least an alpha-helices, cellular PrP and a disease causing form called beta-pleated sheets called prion PrP. All mammals have PrP. Development of prion PrP only occurs in excess production of PrP or due to the formation of prion PrP.

Prion PrP induces the normal cellular PrP into a prion PrP by changing the shape of the cellular PrP.

Prion PrP leads to prion disease such as the mad cow disease or bovine spongiform encephalitis, scrapie, kuru, Creutzfeldt-Jakob disease, among others. These diseases are only transferrable by ingestion of infected tissues and transplanting such organs.

@Michelle:
Prions are pathogens without doubt, but could we call them parasites? As far as I know, parasites are living organisms and they got benefit(s) by living inside / on another living thing. However, prions are merely live-less proteins, and they do not receive benefit of any form (though animals infected are harmed).